Spatially-separated optical filtering system for a laser bar code symbol scanner

ABSTRACT

A laser bar code symbol scanner employing a narrow band-pass optical filtering system of novel construction is disclosed. A first optical filtering element is installed in the light transmission aperture of a bar code scanner housing. A second optical filtering element is installed inside the scanner housing near the light detecting element. The first and second optical filtering elements have wavelength selective properties such that taken together they cooperate to form a narrow wavelength bandpass optical filtering system which allows only transmission of light at and around a certain predetermined wavelength into the photodetector element. The present invention also hides aesthetically unappealing electro-optical components mounted in the scanner housing from plain view and the optical filtering elements of the system can be easily and inexpensively manufactured without compromising the performance of the scanner.

RELATED CASES

[0001] This Application is a Continuation of application Ser. No.08/850,295, filed May 5, 1997, which is a Continuation of applicationSer. No. 08/439,224, filed May 11, 1995, now U.S. Pat. No. 5,627,359,which is a Continuation-in-Part of copending application Ser. No.08/293,491 filed Aug. 19, 1994, now abandoned which is a Continuation ofapplication Ser. No. b 07/761,123 filed Sep. 17, 1991, now U.S. LettersPat. No. 5,340,971, incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to laser scanners used inreading bar and like code symbols, and more particularly to a noveloptical filtering system for use therein, which provides improvedscanner performance, appearance and manufacturability at lower cost.

[0004] 2. Brief Description of the Prior Art

[0005] Laser-based bar code symbol scanning systems have becomeincreasingly popular in recent times. However, despite technicaladvancements in the art, such systems still suffer from numerousproblems that have yet to be adequately solved.

[0006] For example, a major problem with prior art laser scanners isthat as they become more widely used in poinr-of-sale (POS)environments, aesthetic considerations play a greater role in theirpurchase decisions by store managers considering their use at POSlocations. The reason for this is clear. Store owners invest in a greatdeal of time, money and artistic effort in making their stores anddisplay counters attractive to customers. Consequently, store owners andmanagers demand that laser scanning systems do not detract from theappearance of their display and check-out counter environments.

[0007] Another problem with prior art laser scanning systems is that thelaser, mirrors, and other electro-optical components used in suchsystems are revealed to customers at POS locations through opticallytransparent scanning windows. Consequently, the sight of rotatingmirrors and swirling laser beams behind the scanning windows of priorart laser scanners, constitutes a significant source of fear to manycustomers. While such fears are often based on a lack of knowledge oflasers and optics, store managers are nevertheless concerned that suchfears may translate into customer anxiety and thus a decrease in sales.

[0008] Other problems of a more technical nature arise when using priorart laser scanners in POS environments. In particular, typical ambientlighting levels in store environments have the potential of adverselyeffecting the signal-to-noise ratio (SNR) of laser scan data signalsdetected within prior art laser scanners. Thus, to date, a number ofdifferent optical filtering techniques have been developed for use incombating the adverse effects of ambient lighting levels or laserscanner performance. Several optical filtering techniques commonlyemployed are detailed below.

[0009] One popular filtering technique involves installing before thescanner photodetector, a band-pass optical filter narrowly tuned to thelaser wavelength. Typically, this wavelength lies in the visible regionof the electromagnetic spectrum (i.e., about 670 nanometers). Thiscommon filtering technique is used in the prior art laser scanningsystems disclosed in U.S. Pat. Nos. 5,180,904; 5,015,833; 4,816,660;4,387,297 and 5,115,333. However, this approach is not withoutshortcomings and drawbacks. When using this approach, store customersare typically permitted to see the rotating or oscillating mirrors andswirling laser beams behind the scanning window. In addition topresenting a source of worry for many customers, the plain view of suchelectro-optical components also detracts from the overall aestheticappearance of laser scanners employing this common filtering technique.

[0010] Another prior art approach to reducing ambient light in apost-based laser scanners involves installing a spatial filter (i.e., aslotted or aperture plate) over the scanning window of the laserscanner. Typically, the aperture or slot pattern of the aperture platespatially corresponds to the cross-sectional geometry of projected laserscanning pattern at the plane of its scanning window. This spatialfiltering technique is used in the many prior art laser scanningsystems, disclosed in U.S. Pat. Nos. 4,713,532; 4,093,863; and4,647,143. However, this approach is not without its shortcomings anddrawbacks. Such spatial filters detract from the overall appearance ofthe laser scanners in which they are employed. In addition, such spatialfilters cannot be effectively used when the laser scanning patterns arespatially complex, as in the case of the omnidirectional projectionlaser scanner disclosed in U.S. Pat. No. 5,216,232.

[0011] Thus, there is a great need in the art for a laser scanner whichsolves the above-described problems, while overcoming the shortcomingsand drawbacks of prior art laser scanning apparatus and methodologies.

OBJECTS OF THE INVENTION

[0012] Accordingly, it is a primary object of the present invention toprovide a laser bar code symbol scanning system that is capable ofreading bar code symbols, without the shortcomings and drawbacks ofprior art devices.

[0013] A further object of the present invention is to provide a laserbar code symbol scanner having a novel optical filtering system whichprovides improved scanner performance, appearance and manufacturability.

[0014] A further object of the present invention is to provide such alaser bar code symbol scanner, in which the wavelength-selectivecomponents of the optical filter system are strategically installed in aspatially-separated manner in order to achieve improved scannerperformance, appearance and manufacturability, in a simple low-costmanner.

[0015] A further object of the present invention is to provide such alaser bar code symbol scanner in which the optical filtering systememployed therein inherently hides from view, unappealing electro-opticalcomponents mounted within the laser scanner housing, while rejectingunwanted spectral noise outside the narrow spectral band of the laserscanning beam.

[0016] A further object of the present invention is to provide a laserbar code symbol scanner that satisfies the concerns of store owners andmanagers alike, while effectively overcoming the problems caused by highintensity ambient lighting.

[0017] These and further objects of the present invention will becomeapparent hereinafter and in the claims.

SUMMARY OF THE PRESENT INVENTION

[0018] In general, the laser scanner of the present invention provides asimple, low cost solution to the problems described inr the Backgroundof the Invention. This is achieved by strategically embodying a pair ofdiscrete optical filter elements in the housing of a laser scanner inwhich the following system components are provided; a light transmissionwindow; a laser source for producing a laser beam having a predeterminedcharacteristic wavelength; a scanning mechanism for projecting theproduced laser beam through the light transmission window, and scanningthe produced laser beam across a scanning field defined external to thehousing; a laser light focusing means for focusing laser light reflectedoff a scanned bar code symbol, and along a focused laser light returnpath within the housing; and a laser light detection means, disposedalong the focused laser light return path, for detecting the intensityof focused laser light and generating an electrical signalrepresentative thereof.

[0019] In accordance with the present invention, the first opticalfilter element is installed over the light transmission aperture of thescanner housing, and has wavelength selective properties which transmitonly light having wavelengths from slightly below a predeterminedwavelength in the visible band of the electromagnetic spectrum (e.g.,slightly below 670 nanometers and greater). The second optical filterelement is installed within the housing, along the focused laser returnlight path and between the light focusing means and the first opticalfilter element, and transmits only light having wavelengths fromslightly above the predetermined wavelength (e.g., slightly above 670nanometers and greater) . Collectively, the first and second opticalfilter elements cooperate to form a narrow wavelength band-passfiltering system centered about the predetermined wavelength, therebyrejecting wavelengths outside the spectral band of the scanned laserbeam and thus providing improved signal-to-noise ratio.

[0020] As a result of this novel laser scanner construction, thewavelength selective properties of the first optical filter elementinherently render it semi-transparent, and thus hide from plain view,otherwise aesthetically unappealing electro-optical components mountedwithin the scanner housing. At the same time, the second optical filterelement can be made substantially smaller than the size of the lighttransmission window over which the first optical filter element isinstalled, yet still cooperate with the first optical filter element toachieve narrow wavelength band-pass filtering about the characteristicwavelength of the laser beam. Whereas the optical filtering propertiesof the relatively large first optical filter element render itsmanufacture relatively easy and inexpensive, the optical filteringproperties of the relatively small second optical filter element renderits manufacture relatively difficult and expensive. Thus, laser scannerconstruction of the present invention represents a significant advancein the state of the art in laser scanner design and construction.

[0021] In summary, the present invention provides a simple andinexpensive way or making a laser bar code symbol scanner that satisfiesthe concerns of store owners and managers alike, while effectivelyovercoming the problems caused by high intensity lighting conditions inPOS environments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For a fuller understanding of the Objects of the PresentInvention, the Detailed Description of the Illustrated Embodiments willbe taken in connection with the accompanying Drawings, wherein:

[0023]FIG. 1 is a perspective view of a laser bar code symbol readingdevice constructed in accordance with the principles of the presentinvention;

[0024]FIG. 2 is a cross-sectional elevated side view along thelongitudinal extent of the bar code symbol reading device of FIG. 1,showing various hardware and software components used in realizing theillustrative embodiment;

[0025]FIG. 2A is a cross-sectional plan view along with longitudinalextent of the bar code symbol reading device taken along line 2A—2A ofFIG. 2, also showing the various components used in realizing theillustrative embodiment;

[0026]FIG. 3 is a schematic representation of the spectraltransmissioncharacteristics of the first and second optical filter elementsempoloyed in the laser bar code symbol reading device of the presentinvention, graphically illustrating how the spectral transmissioncharacteristics of these spatially-separated optical filter elementscooperate to produce a narrow-band optical filter system centered aboutthe characteristic wavelength of the visible laser scanning beam;

[0027]FIG. 4 is a block functional system diagram of the bar code symbolreading device of the illustrative embodiment of the present invention,illustrating the principal components of the device integrated with thecontrol system thereof;

[0028]FIG. 5 is a perspective view of alternative embodiment of thelaser bar code symbol reading device of the present invention; and

[0029]FIG. 5A is a cross-sectional view of the laser bar code symbolreading device of FIG. 5, taken along line 5A—5A thereof.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

[0030] For purposes of illustration, the present invention will bedescribed below with reference to the accompanying Drawings, with likestructures being indicated by like reference numbers.

[0031] As shown in FIG. 1, automatic bar code symbol reading system 1 ofthe first illustrative embodiment comprises an automatic hand-holdablebar code symbol reading device 2 operably associated with hand-holdabledata collection device 3, described in detail in U.S. Pat. No.5,340,971. Operable interconnection of bar code symbol reading device 2and data collection device 3 is achieved by a flexible multiwireconnector cord 4 extending from bar code symbol device 2 and pluggeddirectly into the data-input communications port of the data collectiondevice 3.

[0032] Referring to FIGS. 1 through 2A, automatic bar code symbolreading device 2 is shown to comprise an ultra-lightweight hand-holdablehousing 5 having a head portion 5A that continuously extends into acontoured handle portion 5B. As illustrated in FIGS. 1 through 3A, thehead portion of housing 5 has a transmission aperture 6 formed in anupper portion of front panel 7 and covered by plastic filter lens 69, topermit laser radiation of a predetermined band of wavelengths, to exitand enter the housing. In general, the lower portion of front panel 7Bis optically opaque, as are all other surfaces of the housing.

[0033] As illustrated in FIG. 1, bar code reading device 2 generates twodifferent fields external to the hand-holdable housing, in order tocarry out automatic bar code symbol reading according to the principlesof the present invention. Specifically, an object detection field,indicated by broken and dotted lines, is provided externally to thehousing for detecting energy reflected off an object bearing a bar code,located within the object detection field. A scan field, on the otherhand, having at least one scanning plant of essentially planar extent,is provided external to the housing for scanning an object presentwithin the scan field. Such scanning is achieved with a laser light beamso that scan data can be collected for detecting the presence of a barcode within the scan field, and subsequently reading (i.e., scanning anddecoding) the detected bar code symbol.

[0034] In general, the energy reflected off an object in the objectdetection field can be optical radiation or acoustical energy, eithersensible or non-sensible by the operator, and may be either generated byan external ambient source, or from the automatic bar code svmbcolreading device itself. In the illustrative embodiment, this energy is abeam of infrared light projected forwardly from transmission aperture 6in a spatially directed fashion, preferably essentially parallel to thelongitudinal axis 9 of the head portion of the housing. In a preferredembodiment, the object detection field has a three-dimensionalvolumetric expanse spatially coincident with the transmitted infraredlight beam. This ensures that an object within the object detectionfield will be illuminated by the infrared light beam and that infraredlight reflected therefrom will be directed generally towards thetransmission aperture of the housing where it can be detected, toindicate that an object is within the object detection field.

[0035] In order to scan a bar code symbol on an object within the objectdetection field, a laser light beam having a characteristic wavelengthλc is automatically generated within the head portion of the housing andrepeatedly scanned through the transmission aperture across the scanfield. As illustrated in FIG. 1, at least a portion of the scanned laserbeam aligned with bar code on the detected object, will be reflected offthe bar code and directed back towards and through the transmissionaperture for collection, detection and subsequent processing in a mannerwhich will be described hereinafter.

[0036] To more fully appreciate the mechanisms employed in providing theobject detection and scan fields of bar code symbol reading device 2,reference is best made to the operative elements within thehand-holdable housing.

[0037] As shown in FIG. 4, bar code symbol reading device of the firstillustrated embodiment comprises a number of system components, namely,an object detection circuit 10, scanning means 11, photoreceivingcircuit 12, analog-to-digital (A/D) conversion circuit 13, bar codepresence detection module 14, bar code scan range detection module 15,symbol decoding module 16, data format conversion module 17, symbolcharacter data storage unit 18, and data transmission circuit 19. Inaddition, a magnetic field sensing circuit 20 is provided for detectinga housing support stand, while a manual switch 21 is provided forselecting long or short range modes of object and bar code presencedetection. As illustrated, these components are operably associated witha programmable system controller 22 which provides a great degree ofversatility in system control, capability and operation. The structure,function and advantages of this controller will be described in detailhereinafter.

[0038] In the illustrative embodiment, system controller 22, bar codepresence detection module 14, bar code scan range detection module 15,symbol decoding module 16, and data format conversion module 17 arerealized using a single programmable device, such as a microprocessorhaving accessible program and buffer memory, and external timing means.It is understood, however, that any of these elements can be realizedusing separate discreet components as will be apparent to those skilledin the art.

[0039] The purpose of the object detection circuit is to determine(i.e., detect) the presence of an object (e.g., product, document, etc.)within the object detection field of bar code symbol reading device 2,and in response thereto, automatically produce first control activationsignal A₁. in turn, first control activation signal A₁ is provided asinput to the system controller which, as will be described in greaterdetail hereinafter, causes the device to undergo a transition to the barcode svmbol presence detection state.

[0040] As illustrated in FIG. 4, scanning means 11 comprises a lightsource 47 which, in general, may be any source of intense light suitablyselected for maximizing the reflectivity from the object's surfacebearing the bar code symbol. In the illustrative embodiment, lightsource 47 comprises a solid-state visible laser diode (VLD) which isdriven by a conventional driver circuit 48. In the illustrativeembodiment, the wavelength of laser light produced from laser diode 47is about 670 nanometers. In order to scan the laser beam output fromlaser diode 47 over a scan field having a predetermined spatial extentin front of the head portion of the housing, a planar scanning mirror 49can be oscillated back and forth by a stepper motor 50 driven by aconventional driver circuit 51, as shown. However, it is understood thatother conventional laser scanning mechanisms may be used to practice thepresent invention.

[0041] To selectively activate laser light source 47 and scanning motor50, the system controller provides laser diode enable signal E_(L) andscanning motor enable signal E_(M) as input to driver circuits 48 and51, respectively. When enable signal E_(L) is a logical “high” level(i.e., E_(L)=1) , a laser beam is generated, and when E_(M) is a logicalhigh level the laser beam is scanned through the transmission apertureand across the scan field.

[0042] When an object such as product bearing a bar code symbol ispresented within the scan field at the time of scanning, the laser beamincident thereon will be reflected. This will produce a laser lightreturn signal of variable intensity which represents a spatial variationof light reflectivity characteristic of the spaced apart pattern of barscomprising the bar code symbol. Photoreceiving circuit 12 is providedfor the purpose of detecting at least a portion of laser light ofvariable intensity, which is reflected off the object and bar codesymbol within the scan field, and subsequently focused along a focusedlaser light return path within the housing, onto the photosensor ofphoto-receiving circuit 12. Upon detection of this scan data signal,photoreceiving circuit 12 produces an analog scan data signal D₁indicative of the detected light intensity.

[0043] In the illustrative embodiment, photoreceiving circuit 12generally comprises scan data collection optics 53, which focus opticalscan data signals for subsequent detection by a photoreceiver 54 having,mounted in front of its sensor, a wavelength-selective filter 150 whichonly transmits optical radiation of wavelengths up to a small band above670 nanometers, as illustrated in FIG. 3. Photoreceiver 54, in turn,produces an analog signal which is subsequently amplified bypreamplifier 55 to produce analog scan data signal D₁. In combination,scanning means 11 and photoreceiving circuit 12 cooperate to generatescan data signals from the scan field, over time intervals specified bythe system controller. As illustrated hereinafter, these scan datasignals are used by bar code presence detection module 14, bar code scanrange detection module 15 and symbol decoding module 16.

[0044] As illustrated in FIG. 4, analog scan data signal D₁, is providedas input to A/D conversion circuit 13. As is well known in the art, A/Dconversion circuit 13 processes analog scan data signal D₁ to provide adigital scan data signal D₂ which resembles, in form, a pulse widthmodulated signal, where logical “1” signal levels represent spaces ofthe scanned bar code and logical “0” signal levels represent bars of thescanned bar code. A/D conversion circuit 13 can be realized bv anyconventional A/D chip. Digitized scan data signal D₂ is provided asinput to bar code presence detection module 14, bar code scan rangedetection module 15 and symbol decoding module 16.

[0045] The purpose and function of bar code presence detection module 14is to determine whether a bar code is present in or absent from the scanfield over time intervals specified by the system controller. When a barcode symbol is detected in the scan field, the bar code presencedetection module 14 automatically generates second control activationsignal A₂ (i.e., A₂=1) which is provided as input to the systemcontroller, as shown in FIG. 4. Preferably, bar code presence detectionmodule 14 is realized as a mircrocode program carried out by themicroprocessor and associated program and buffer memory, describedhereinbefore. The function of the bar code presence detection module isnot to carry out a decoding process but rather to simply and rapidlydetermine whether the received scan data signals produced during barcode presence detection, represent a bar code symbol residing within thescan field. There are many ways in which to realize this functionthrough a programming implementation.

[0046] When a bar code symbol envelope is detected, the bar code symbolpresence detection module provides second control activation signal A₂=1to the system controller. As will be described in greater detailhereinafter, second control activation signal A₂ =1 causes the device toundergo a transition from the bar code presence detection state to barcode symbol reading state.

[0047] The function of symbol decoding module 16 is to process, scanline by scan line, the stream of digitized scan data D_(2,) in anattempt to decode a valid bar code symbol within a predetermined timeperiod allowed by the system controller. When the symbol decoding modulesuccessfully decodes a bar code symbol within the predetermined timeperiod, symbol character data D₃ (typically in ASCIII code format) isproduced corresponding to the decoded bar code symbol. Thereupon a thirdcontrol activation signal A₃ is automatically produced by the symboldecoding module and is provided to the system controller in order toperform its system control function.

[0048] As shown in FIG. 4, system controller 22 generates and providesenable signals E_(EC,) E_(DS,) E_(DT) to data format conversion module17, data storage unit 18 and data transmission circuit 19, respectively,at particular stages of its control program. As illustrated, symboldecoding module 16 provides symbol character data D₃ to data formatmodule 17 to convert data D₃ into two differently formatted types ofsymbol character data, namely D₄ and D₅. Format-converted symbolcharacter data D₄ is of the “packed data” format, particularly adaptedfor efficient storage in data storage unit 18. Format-converted symbolcharacter data D₅ is particularly adapted from data transmission to datacollection and storage device 3, or a host device such as, a computer orelectronic cash register. When symbol character data D₄ is to beconverted into the format of the user's choice (based on a selectedoption mode), the system controller will generate and provide enablesignal E_(DS) to data storage unit 18, as shown in FIG. 4. Similarly,when format converted data D₅ is to be transmitted to a host device, thesystem controller will generate and provide enable signal E_(DT) to datatransmission circuit 19. Thereupon, data transmission circuit 19transmits format-converted symbol character data D₅ to data collectiondevice 3, via the data transmission lines of flexible connector cable 4.

[0049] It is understood that there are a variety of ways in which toconfigure the above-described system components within the housing ofbar code symbol reading device 2, while successfully carrying out thefunctions of the present invention. In FIGS. 2 and 2 A, one preferredarrangement is illustrated.

[0050] In FIG. 2A, the optical arrangement of the system components isshown. Specifically, visible laser diode 47 is mounted in the rearcorner of circuit board 64 installed within the head portion of thehousing. A stationary concave mirror 53 is mounted centrally at thefront end of circuit board 63, primarily for collecting laser light.Notably, the height of concave mirror 53 is such that it does not blocklight transmission aperture 6. Mounted off center onto the surface ofconcave mirror 53, is very small second mirror 64 for directing thelaser beam to planar mirror 49 which is connected to the motor shaft ofa scanning motor 50, for joint oscillatory movement therewith. As shown,scanning motor 50 is mounted centrally at the rear end of circuit board63. In the opposite rear corner of circuit board 63, photodetector 54 isnounted.

[0051] In operation, laser diode 47 adjacent the rear of the headportion, produces and directs a laser beam in a forward direction to thesmall stationary mirror 64 and is reflected back to oscillating mirror49. Osc illating mirror 49 scans the laser beam over the scan field. Thereturning laser light, reflected from the bar code, is directed back tooscillating mirror 49, which also acts as a collecting mirror. Thisoscillating mirror then directs the beam to stationary concave mirror 53at the forward end of the housing head portion. The beam reflected fromthe concave mirror 53 is directed to photodetector 54 to produce anelectrical signal representative of the intensity of the reflectedlight.

[0052] In front of stationary concave mirror 53, IR LED 28 andphotodiode 31 are mounted to circuit board 63 in a slightly offsetmanner from longitudinal axis 9 of the head portion of the housing.Apertures 65 and 66 are formed in opaque portion 7B of the housing belowthe transmission aperture, to permit transmission and reception of IRtype object sensing energy, as hereinbefore described. In order toshield IR radiation from impinging on photodiode 31 via the housing, ametallic optical tube 67 having an aperture 68 encases ohotod iode 31.By selecting the size of aperture, the placement of photodiode 31 withinoptical tube 67 and/or the radiation response characteristics of thephotodiode, desire geometric characteristics for the object detectionfield can be achieved, as described hereinbefore.

[0053] To prevent optical radiation slightly below 670 nanometers fromentering the transmission aperture 6, and transmitting therethrough onlyoptical radiation from slightly below 670 nanometers, a plastic filterlens 69 is installed over the transmission aperture 6, as shown inFIG. 1. In this way the combination of plastic filter lens 69 installedat the transmission aperture and the wavelength selective filter 150mounted before photoreceiver 54, as shown in FIG. 2A, cooperate witheach other in terms of wavelength selection characteristics, to form anarrow band-pass optical filter system having a center wavelengthμ_(c)=670 nanometers, as shown in FIG. 3.

[0054] In the illustrative embodiment, plastic window filter lens 69 ismade from acrylic-type plastic material (e.g., DuPont RD 2177) which canbe purchased in 4′×8′sheets. These acrylic sheets are cut to size so asto fit over the light transmission aperture 6. The resulting plasticfilter lens 69 is then installed into the light transmission aperture ina manner well known in the art.

[0055] Wavelength-selective filter 150 is preferably made by coating(i.e., depositing) a multi-layer dielectric film onto a glass substrate.In a vacuum environment (i.e., chamber), the dielectric film ispreferably deposited onto the glass substrate by evaporating adielectric material with an electric beam, in a manner well known in theart. Thereafter, the resulting substrate with the dielectric filmdeposited thereon is cut into small pieces having physical dimensionsapproximately the size of the photosensor in photoreceiver 2, as shownin FIGS. 2 and 2A, thereby providing wavelength-selective filter 150.The wavelength-selective filter 150 is then mounted immediately in frontof the photosensor, as shown in FIGS. 2 and 2A.

[0056] The novel optical filter arrangement described above provides anumber of important advantages to the laser scanner in which it isembodied.

[0057] Firstly, the narrow-band optical filter system of the presentinvention rejects wavelengths outside the narrow-band of spectralcomponents comprising the laser scanning beam (i.e. associated withambient light noise), and this improves the signal-to-noise ratio fordetected scan data signals D₁.

[0058] Secondly, the spectral filtering characteristics of plasticfilter lens 69 inherently appears reddish to the human vision system byvirtue of the fact that lens 69 only permits transmission of opticalradiation from slightly below 670 nanometers. Thus, the semi-transparentnature of filter lens 69 naturally hides from plain view, the laser, themirror, the scanning motor, and other electro-optical components withinthe housing that otherwise might present source of fear in customers ata POS station, and/or detract from the aesthetic appearance of thescanning system installed at POS station.

[0059] Thirdly, the plastic filter lens 69 with its specified opticalproperties is easy and inexpensive to manufacture using injectionmolding techniques well known in the art. Thus, it may be made as largeas desired or formed (i.e., shaped) to embody beam-shaping orbeam-directing characteristics, without substantially increasing thecost of manufacture of this optical filter element.

[0060] Wavelength-selective filter element 150, on the other hand, isvery expensive and difficult to manufacture, by virtue of its specifiedoptical properties. However, as this optical filter element 150 isinstalled along the focused laser light return path, in front ofphotoreceiving sensor 54 as shown in Fig. 2A, its size can be maintainedextremely small, independent of the surface area of the lighttransmission aperture, and thus the plastic filter lens 69.Consequently, conventional techniques can be used to manufacture thissmall-sized optical filter element, and thus the cost of manufacture ofthis optical element can be minimized.

[0061] Fourthly, by using spatially-separated optical filter elements(i.e., plastic filter lens 69 and filter element 150), the use ofspecial optical cements and bonding techniques otherwise required tophysically bound such elements together in an integral filter structure,are avoided altogether. This fact simplifies significantly themanufacturability of the laser scanner of the present invention.

[0062] The optical filter system described above may be embodied in anytype of laser bar code symbol scanner. An example of such an alternativelaser scanner design is shown in FIGS. 5 and 5A.

[0063] In FIGS. 5 and 5A, the optical filter system of the presentinvention is shown embodied in the laser projection scanner disclosed inU.S. Pat. No. 5,216,232. As disclosed in FIGS. 5 and 5A, plastic filterelement 69′ is functionally similar to optical filter element 69 andcovers the light transmission aperture of the compact housing of thelaser projection scanner, while wavelength selective filter 150′ isdisposed in front of its photodector 110 along the focused laser lightreturn path defined between light focusing mirror 120 and photodector110, as shown in FIG. 5A. By virtue of the principles of the presentinvention, plastic filter element 69′ over light transmission aperture6′ can be made substantially larger than wavelength selective filter150′, as required in practical scanner designs, yet it provides all ofthe advantages described above.

[0064] In alternative laser scanner designs the alternate optical filtersystem disclosed herein may be embodied within laser holographicscanners used to read code symbols in various applications.

[0065] While the particular illustrative embodiments shown and describedabove will be useful in many applications in code symbol reading,further modifications to the present invention herein disclosed willoccur to persons skilled in the art. All such modifications are deemedto be within the scope and spirit of the present invention defined bythe amended claims.

What is claimed is:
 1. A laser symbol scanning system, cpmprising: ahousing having a light transmission aperture through which light canenter and exit said housing; a first optical filter element disposed insaid light transmission aperture and along a laser light path extendingthrough said light transmission aperture, and havingwavelength-selective filtering characteristics, said first opticalfilter element function as a scanning window in said housing; laser beamproducing means in said housing for producing a laser beam characterizedby a predetermined wavelength; laser beam scanning means in said housingfor projecting said laser beam through said scanning window and scanningsaid laser beam across a code symbol on an object located within atleast a portion of a scan field defined external to said housing; laserlight detection means is said housing disposed in said laser light path,for detecting the intensity of laser light reflected off said codesymbol; and a second optical filter element in said housing, spatiallyseparated from said first optical filter element and disposed along saidlaser light path between said first optical filter element and saidlaser light detection means, and having wavelength-selective filteringcharacteristics, said second optical filter element cooperating withsaid first optical filter element so as to form a band-pass opticalfiltering system having a narrow wavelength band width positioned aboutsaid predetermined wavelength and passing light reflected off said codesymbol having wavelengths only within said narrow wavelength bandwidth.2. The laser code symbol scanning system of claim 1 , wherein said firstoptical filtering element prevents light having wavelengths up toslightly below said predetermined wavelength from passing through saidfirst filter.
 3. The laser code symbol scanning system of claim 1 ,wherein said second optical filtering element transmits light havingwavelengths from slightly above said predetermined wavelength.
 4. Thelaser code symbol scanning system of claim 1 , wherein saidpredetermined wavelength is about 670 nanometers.
 5. The laser codesymbol scanning system of claim 2 , wherein said predeterminedwavelength is about 670 nanometers.
 6. The laser code symbol scanningsystem of claim 3 , wherein said predetermined wavelength is about 670nanometers.
 7. The laser code symbol scanning system of claim 1 whereinsaid laser beam producing means comprises a visible laser diode forproducing a visible laser beam.
 8. The laser code symbol scanning systemof claim 1 wherein said second optical filtering element is disposedimmediately adjacent said light detecting means.
 9. The laser codesymbol scanning system of claim 1 wherein said wavelength filteringcharacteristics of said first optical filtering element obscures each ofsaid means positioned in said housing from plain view.
 10. The lasercode symbol scanning system of claim 1 wherein in said housing is acompact hand-supportable housing.
 11. The laser code symbol scanningsystem of claim 1 wherein said housing is mounted above a countertop.12. The laser code symbol scanning system of claim 1 wherein said laserbeam scanning means produces a single-line laser scanning pattern. 13.The laser code symbol scanning system of claim 1 wherein said laser beamscanning means produces an omnidirectional laser scanning pattern.